The subject invention relates to improvements in soles designed for use with sports shoes, particularly tennis shoes, said sole comprising an insole, a midsole of a comparatively soft compressible and resilient material, and an outsole.
Some sports are mostly practiced on non-resilient surfaces, such as asphalt surfaces and concrete and wooden floors. One example of such sports is tennis. In the practice of this sport it is usually necessary for the player to move in all directions, alternating between rapid accelerating movements and sudden standstills. Movements of this kind on non-resilient surfaces put considerable stress above all on the ankles of the players.
Traditionally, sports shoe soles are made from rubber. This material is resilient and possesses a high coefficient of friction. However, the high weight of rubber is a drawback when used for soles and so is its "rebouncing" property, that is, it absorbs only a minor portion of the energy of impact when the foot hits the ground. This is a considerable disadvantage when the flooring surface is unresilient.
It is already known to provide sports shoes designed for playing on surfaces of this kind with a midsole of polyurethane plastics. Like rubber, this is a resilient material but in addition it possesses good shock-absorbing properties and also it is lighter than rubber. Compared with rubber, polyurethane has higher strength and wear-resistance but its frictionel properties are poorer.
For the purpose of improving the shock-absorbing qualities and increase the wear and strength of the sole the sports shoes manufactured today use polyurethane plastics for the midsole as well as for the outsole, the outsole being, however, slightly harder in order to provide sufficient wear resistance. Soles of this kind have proved to suffer from the disadvantage of being somewhat slippery when used in connection with certain types of surface, i.e. they do not provide sufficient grip or friction on the surface underfoot.